Method of casting composite cooking vessel



June 1, 1965 A. LAGOSTINA METHOD OF CASTING COMPOSITE COOKING VESSEL Filed Nov. 29. 1960 NVENTOR. mm

United States Patent 4 (Claims. in. 22--2tl4) This invention relates to the coating of metals with other metals, and has particular reference with the coating of the bottom surface of stainless steel made cooking vessels with an aluminum or its alloys to form a heat transmitting layer of substantial thickness integrally connected to the outer face of the bottom portion of said vessels.

It is currently known that stainless steel made cooking vessels, namely pots, pans, frying-pans, sauce-pans and the like, may be greatly improved by applying to the outer face of the bottom portion thereof a layer of substantial thickness of aluminum or of aluminum based alloy which possesses a high heat conductivity, in view of improving the heat absorption characteristic of the cooking vessel and improve the uniform distribution of heat throughout the bottom area of the cooking implement;

Such light metal made heat conductive layer should obviously be fixedly secured to the adjacent portion of the stainless steel made structure to form therewith a composite structure which would withstand severe stresses, mechanical deformation, wide and abrupt variation temperature, andthe like, without separation at the zone between the two metals. In addition, the outer face and edge portion of the light metal coated vessel would'be provided with satisfyingly fiat and smooth surfaces.

Heretofore various methods have been proposed for coating the bottom portion of stainless steel made cooking articles of the type considered, but few of these methods have been sufficiently satisfactory to justify their commercial use for all purposes. Also, these prior methods were complicated and expensive and the results of their use were not fully satisfactory either in view of efficiency and durableness of the bond between the adjacent metals and in view of the compactness of the light metal made heat conductive layer These prior methods generally involved a preliminary step for making the outer surface of stainless steel adapted for bonding with the aluminum, a coating step whereby the said layer of substantial thickness is formed on and integrally connected with said prepared outer surface, and a finishing step whereby the coated article of manufacture is fully machined at its light metal coated portion for providing same with the desired properly shaped and smoothly finished outer surfaces. The said preliminary step of prior methods includes, in general, the forming of thin layer of aluminum on the thoroughly cleaned surface of the stainless steel, by dipping the uncoated vessel in molten aluminum, or by spraying the stainless steel surface with molten aluminum, said thin layer being proved as adapted for a substantially satisfactory and integrally bonding thereof the said layer of substantial thickness.

The said second or coating step of the prior methods includes the forming of the aluminum or aluminum based alloy made layer of the desired thickness, such layer being generally applied by casting the molten aluminum or alloy on the article, preferably by placing a mold about the previously coated surface of the article and pouring molten aluminum in contact therewith to the desired thickness. When the aluminum solidifies, a substantially homogeneous layer of aluminum is provided on the surface of the article which is secured by the bond of the said previously applied thin preliminary coating. This relatively c lC thick layer of aluminum must be then machined by placing the article on a lathe or otherwise mechanically treated to produce the finished product for commercial purpose.

The said above methods are subject to the objection that the machining of molten and solidified layer of aluminum is rather expensive and, in addition, may cause the said layer to be separated from the base stainless steel structure of the cooking vessel, and that the cast aluminum may not be compact, homogeneous and poreless as desirable.

Having the above considerations and objections in mind, it therefore is an object of this invention to provide a new and advantageous method for producing composite cooking articles as referred to above, which is not subject to the above and other objections and by which a better and more desirable article may be manufactured.

More particularly, it is an object of this invention to provide a new and improved method of manufacture which is conducted essentially in three steps, beginning with a thorough cleaning of the outer bottom surface of the base stainless steel made cooking vessel structure and preferably with making such surface best adapted for bonding with aluminum, namely by sand-blasting or otherwise roughening said surface. The second step consists of immediately providing such cleaned and preferably roughened surface with a first thin layer of aluminum or of aluminum based alloy adapted to embody a preliminary coated surface on which the layer of substantial thickness may be integrally bonded. The third step consists in producing and applying the said substantially thick layer of aluminum or of aluminum based alloy on said previously coated surface, by pressure die casting the light metal on and about the said surface, namely by placing a pressure resisting die mold on and about the said previously coated surface of the article and defining together with said surface a closed space dimensioned and shaped to produce the finished coating thereinto, by supporting the bottom portion of the stainless steel made vessel by means of a counter-die placed thereinto, and by injecting pressurized molten aluminum or aluminum based alloy in said closed space.

The new features of the invention are set forth in the appended claims. The present invention, both as to the method for carrying on the same and as to further objects and advantages thereof, will be however best understood by way of illustration and example by the following detailed description of a preferred form 'of embodiment thereof, when taken in conjunction with the accompanying drawing, forming an essential component of this disclosure, and wherein:

FIGURE 1 is partly a side elevation and partly a vertical sectional view of a cooking vessel, namely of a stainless steel made pot provided with a heat conductive coating produced according ,to the invention;

FIGURE 2 diagrammatically illustrates the step of producing the preliminary thin coating on the bare stainless steel surface, according to a preferred form of embodiment of the invention; and

FIGURE 3 diagrammatically illustrates the step of die casting the heat conductive and substantially thick coating and the apparatus provided therefor.

Referring first to FIG. 1 of theaccompanying drawing, there is shown a cooking implement, namely a cooking pot 16 made of stainless steel sheet metal, and produced by applying current knowledge of the art to which this invention appertains.

Such pot It) includes a flat bottom portion ll. having rounded edge portions at 15 in flush with the side portions of the pot. The shape of such cooking implement may be obviously varied according to its particular use, name- 1y according to the use of article say as a pan, for example.

Such stainless steel made article is provided with a heat conductive light metal made bottom outer layer 12, integrally connected to said bottom portion 11 and preferably with part of said rounded edge portions 15. Such heat conductive layer may be made of pure aluminum or of an aluminum based alloy, if desired. Such outer layer 12 is of substantial thickness; for example, such layer may be 2 millimeters to 8 millimeters thick. The thickness of such layer 12 is matter of choice and depends from various considerations, either in view of the particular cooking use of the article and of the desired cost of the finished article on sale.

In addition, the said outer layer 12 is provided with an essentially fiat and smooth outer face 13 and with properly shaped and smooth edge portions 44.

For producing and applying the said layer 12 of heat conductive light metallic material appertaining to a group including pure or substantially pure (99 percent) aluminum or aluminum based alloys, the method includes a first step consisting of a preferably but not necessarily chemical thorough cleaning of the outer surface of said bottom portion 11 and of the adjacent rounded portions at 15 of the stainless steel made article, followed by a substantial roughening of said cleaned surface, say by sand-blasting. Such mechanical roughening action may be advantageously substituted by a chemical slightly erosive action exerted by the chemically cleaning agent, if desired. It is believed that no further description is necessary for making apparent such first step of the method, in consideration of the fact that such cleaning and roughening procedure is Well known to those skilled in the art.

The second step of the method is diagrammatically shown in FIG. 2. Such second step is preferably performed immediately after the said first step, for preventing oxidation of the bare metal. Such second step is directed to provide the said outer surface of bottom portion of the article with a thin layer 16 of light metal for making the outer bottom surface adapted for inimately and integrally forming and connecting the layer 12 of substantial thickness thereon.

Such second step is preferably performed by spraying droplets of molten light metal on the cleaned surface, the said spraying operation being diagrammatically indicated in FIG. 2 by a plurality of arrows 17. Such spraying essentially embodies the utilization of a well known procedure of pressure spray-metallization, and it may be performed by applying the current techniques and tools of such procedure.

According to the invention, however, the said spray procedure may be preferably improved by performing the same spraying on pre-heated stainless article, heated in an oven to a temperature slightly lower than the melting temperature of the light metal or alloy to be sprayed. In providing the said first thin coating 16 with substantially pure aluminum, the bare stainless steel made article is preferably heated at 450 to 500 C. and most preferably to 480 C., to ensure the most desirable inimate bond of the light metal to the poorly heat conductive stainless steel.

The third step of the method of the invention is the most critical of the invention itself and leads to new and unexpected results. Such step is performed by making use of a proper high pressure die-casting apparatus produced and operated according to current art and including a die mold 18 adapted to be applied to the bottom portion 11 of the cooking vessel to be coated and defining therewith a closed space 19 an inner surface of which is formed by the said bottom portion 11 previously provided with the thin coating 16, the thickness and the shape of said space 19 corresponding to the desired thickness and shape of the finished layer 12 (FIG. 1) to be produced.

The said layer is produced by injecting pressurized molten light metal 20 into said space 19 by means of a suitable die casting injector generally indicated at 21, while the bottom portion 11 of the article thoroughly abuts on a counter-die 22 snugly fitting inside the cooking vessel, whereby deformation of said bottom portion 11 by the remarkable pressure exerted by the pressurized molten metal injected into said chamber 19 is fully prevented. The injection of the said pressurized molten metal 20 into the said chamber 19 is made via a small sized nozzle passage 23 provided in the die mold wall, preferably at the centre thereof, whereby the obtained die-cast layer 12 will not retain substantial discards or dead head portions to be removed therefrom.

The substantial mass of the molten material pressurizedly injected in the chamber 19 makes not strictly necessary a pre-heating of the article to be provided with the desired coating 12, owing to the high specific heat of aluminum, but the pre-heating of the article at least at C. has been found advantageous for improving the intimate bond of the aluminum to stainless steel. Such pre-heating may be made not necessary by immediately transferring the article from the spray-metallization apparatus of second step to the pressure molding apparatus, so that sufiicient heat is retained in the article for improving the result of third step, as above described.

As a result of such step, a surprising not porous and uniform coating 12 may be produced, capable to withstand the most severe mechanical and thermal shocks, and provided with a properly shaped, flat and smooth surface, so that further machining of the article may be useless, a simple smoothing and finishing operation having been proved as suflicient for imparting to the article the most desirable appearance for the sale thereof.

It is believed to be evident that the invention provides a plurality of advantageous features, and it will be understood that each of the new features described and shown and any combination thereof may also find useful application in other methods for producing composite articles differing from the one described.

Without further consideration, the foregoing will so fully reveal the gist of this invention that others can, by applying current knowledge, readily adapt it for other applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of this invention and, therefore, such adaptations should and are intended to be comprehended within the spirit and meaning of this invention as above set forth and defined in and by the appended claims.

Having thus described the invention, what I claim as new and desire to have protected by Letters Patent of the United States of America is:

1. A method of forming a heat-conductive coating on the outer surface of the bottom portion of a stainless steel cooking vessel, comprising the steps of cleaning the outer surface of said bottom portion; preheating at least said bottom portion of said cooking vessel to a temperature ranging from 450 C. to 500 C.; spraying said outer'surface of said bottom portion with an aluminum-containing metal selected from the group consisting of aluminum and aluminum alloys so as to produce an initial relatively thin, aluminum containing coating thereon; and die-casting under pressure on said initially coated surface a layer of a greater thickness, and of aluminum-containing metal selected from the group consisting of aluminum and aluminum alloys.

2. A method of forming a heat-conductive coating on the outer surfaces of the flat bottom portion and adjacent rounded edge portion of a stainless steel cooking vessel, comprising the steps of cleaning the outer surfaces of said bottom and edge portions; preheating at least said bottom and edge portions of the cooking vessel to a temperature ranging from 450 C. to 500 C.; spraying said outer surfaces of said bottom and edge portions with substantially pure aluminum to produce an initial filmlike, aluminum coating thereon; and die-casting under pressure on said initially coated surfaces a layer of a 5 substantially greater thickness and of aluminum-containing metal selected from the group consisting of aluminum and aluminum alloys.

3. The method set forth in claim 2, wherein a thickness ranging from about 2 millimeters to about 8 millimeters is imparted tothe die-cast layer.

4. The method set forth in claim 2, including the step of supporting the bottom portion of the cooking vessel at the inside thereof during the die-casting operation.

References Cited by the Examiner UNITED STATES PATENTS Schaefer 22--200.l

Ladd 22.204

Patera 18-30 Jepson 22-203 Ferrell et al 1817 FOREIGN PATENTS Great Britain.

OTHER REFERENCES Metals Handbook, ASTM, 1948, pages 720-721.

MICHAEL V. BRINDISI, Primary Examiner.

WINSTON A. DOUGLAS, NEDWIN BERGER,

Examiners. 

1. A METHOD OF FORMING A HEAT-CONDUCTIVE COATING ON THE OUTER SURFACE OF THE BOTTOM PORTION OF A STAINLESS STEEL COOKING VESSEL, COMPRISING THE STEPS OF CLEANING THE OUTER SURFACE OF SAID BOTTOM PORTION; PREHEATING AT LEAST SAID BOTTOM PORTION OF SAID COOKING VESSEL TO A TEMPERATURE RANGING FROM 450*C. TO 500*C; SPRAYING SAID OUTER SURFACE OF SAID BOTTOM PORTIONS WITH AN ALUMINUM-CONTAINING METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS SO AS TO PRODUCE AN INITIAL RELATIVELY THIN, ALUMINUM CONTAINING COATING THEREON; AND DIE-CASTING UNDER PRESSURE ON SAID INITIALLY COATED SURFACE A LAYER OF A GREATER THICKNESS, AND OF ALUMINUM-CONTAINING METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS. 